1. Field of the Invention
The present invention relates to an image forming apparatus for transferring, to a transferring material such as paper or a plastic sheet, a transferable image formed on a first image bearing body such as an electrophotographic photosensitive body, an electrostatic recording dielectric body or a magnetic recording magnetic body by a known image forming process means such as electrophotography, an electrostatic recording method or a magnetic recording method.
2. Related Background Art
FIG. 7 is a schematic diagram showing an example of transferring apparatus in a conventional image forming apparatus. This transferring apparatus is of a roller transferring type.
Reference numeral 1 denotes a rotating drum type electrophotographic photosensitive body (hereinafter referred to as a photosensitive drum) which is used as a first image bearing body. This photosensitive drum 1 is rotatingly driven in a clockwise direction indicated by an arrow at a predetermined peripheral speed (process speed) and a toner image corresponding to a target image information is formed on an outer circumferential surface of the photosensitive drum 1 as a transferable image (visualized image) by an operation of an electrophotography process appliance (not shown).
Reference numeral 5 denotes an electrically conductive elastic roller (herein after referred to as a transferring roller) which is used as transferring means (contact transferring member). This transferring roller 5 is disposed in parallel with the photosensitive drum 1, pressed to the photosensitive drum 1 at a transferring location under a predetermined pressure so as to form a transferring nip portion N and rotatingly driven in a counterclockwise direction indicated by an arrow which is a forward direction of the photosensitive drum 1 at a predetermined peripheral speed nearly corresponding to the rotating peripheral speed of the photosensitive drum 1.
Reference character P denotes a transferring material which functions as an image bearing body. This transferring body P is fed from a sheet feeding portion (not shown) and conveyed at a predetermined controlled timing to the transferring nip portion N which is a pressure contact portion between the photosensitive drum 1 and the transferring roller 5. In other words, a top of the transferring material P is detected with a sensor 8 and a timing is adjusted so that a toner image forming position on the photosensitive drum 1 is matched with a writing start position on the top of the transferring material P.
The transferring material P which is conveyed at the predetermined timing to the transferring nip portion N is inserted under a predetermined pressure and conveyed in the transferring nip portion N, electric charges having a polarity reverse to that of a toner are imparted to a rear surface of the transferring material P by a function of a bias voltage applied to the transferring roller 5 from a power source 9 by way of a roller core metal and a toner image on the photosensitive drum 1 is transferred to the transferring material P with these electric charges.
After transferring the toner image, excessive electric charges are removed from the rear surface of the transferring material P using an antistatic wire or the like, the transferring material P is sent into a fixing apparatus (not shown) while bearing the transferred toner image and the toner image is fixed permanently on the transferring material P.
After the transferring material P has passed through the transferring nip, a surface of the photosensitive drum 1 is cleaned by wiping off the toner remaining after transferring the toner image using a cleaning apparatus (not shown) and used once again for forming an image.
The applicant of the present invention has already proposed an active transfer voltage control system (hereinafter referred to as ATVC system) which is capable of controlling the bias voltage to the above described transferring roller 5 so that favorable transferring performance can be always obtained regardless of changes in environmental conditions (Japanese Patent Application Laid-Open No. 2-123385).
Specifically, this ATVC system rotates the photosensitive drum 1 prior to an image forming step (preliminary rotation), applies the bias voltage to the transferring roller 5 during the preliminary rotation, measures an output current value at this time with an ammeter 10 and feeds back a measured value to a controller 11. The ATVC system adjusts the bias voltage from the power source 9 with the controller 11 so that the above described output current value is a predetermined value and applies an adjusted voltage or a constant voltage having a value corrected with a coefficient or the like to the transferring roller 5 at a transferring time, thereby making it possible to always obtain a transferring bias voltage having an appropriate constant voltage characteristic regardless of remarkable variations of impedance of the transferring roller 5 independently of the environment.
Since the above described conventional system adjusts a constant bias voltage to be applied to the transferring roller 5 so that the current has the predetermined value in a condition where the photosensitive drum 1 is in direct contact with the transferring roller 5, however, the conventional system has a defect that is causes improper transferring in cases where:
1) The transferring material P has high impedance (for example, in a case where a thick sheet is used or a print is made on a rear surface of the transferring material P which is used once for printing); and
2) The transferring roller 5 has low impedance.
This defect will be described using a figure of voltage-current characteristic curve of the power source for applying the bias voltage to the transferring roller 5 shown in FIG. 8.
In FIG. 8, a curve A represents relationship between a bias voltage V to the transferring roller 5 and an output current I when the photosensitive drum 1 and the transferring roller 5 are rotated in a direct contact condition, and a voltage Va is determined in this case so as to obtain an output current Ia during the preliminary rotation and used as a constant bias voltage to the transferring roller 5 at a transferring step during image formation.
When paper (a transferring material-1) is used as the transferring material P, a V-I characteristic curve is a curve P1 in a condition where the above described transferring material P1 is inserted in a transferring nip portion N between the photosensitive drum 1 and the transferring roller 5, whereby application of the constant bias voltage Va produces a transferring current I1. It may be questioned whether the transferring current I1 is sufficient, but the toner image is transferred favorably in this case since the transferring current I1 is higher than a critical transferring current value It as shown in FIG. 8.
When a transferring material having high impedance, for example, thick paper (a transferring material-2) is used, however, the V-I characteristics is as represented by a curve P2 which is nearer a V axis and the bias voltage Va produces only a transferring current I2 which is lower than It, thereby causing improper transferring.
Furthermore, a curve Axe2x80x2 in FIG. 8 represents a V-I characteristic in a case where impedance of the transferring roller 5 is lower than that represented by a curve A, and in this case, a voltage corresponding to the predetermined current value Ia during the preliminary rotation is Vaxe2x80x2 and a constant bias voltage which is to be applied at the transferring time is Va. In this case, a transferring current for the transferring material-1 is also lower than the critical transferring current value It, thereby causing improper transferring.
When the transferring roller 5 has rather low impedance, the curves P1 and P2 corresponding to the curve Axe2x80x2 are actually represented as curves which are slightly farther from the V axis, but these curves are different only slightly from the curves P1 and P2 shown in FIG. 8 and not shown for simplicity of description.
Though it is necessary for maintaining transferring performance to supply electric charges sufficiently to the transferring material P, that is, to maintain the current values I1 and I2 at levels not lower than It, the conventional ATVC system is configured on a premise that the current value Ia during the preliminary rotation is in a definite proportional relation to the current value I1 (or I2) at the transferring time and inevitably causes improper transferring as described above when the impedance of the transferring material P or the transferring roller 5 changes.
In order to solve a problem such as that described above, Japanese Patent Application Laid-Open No. 4-251276 or the like discloses a method for obtaining a transferring apparatus which is configured not to cause improper transfer.
This method is configured to measure an output current from the power source 9 with the ammeter 10 in a condition where the transferring material P is inserted in the transferring nip portion formed by the photosensitive drum 1 and the transferring roller 5 (in a condition where a top of the transferring nip portion in particular), feed back a measured current to a controller 11 and control a bias voltage of the power source 9 so that the above described output current has a predetermined value, thereby preventing improper transferring regardless of the impedance of the transferring material P and the transferring roller 5.
The output current from the power source 9 is measured with the ammeter 10 while the above described transferring material-2 having the high impedance, for example, moves for a distance Le from the top in the moving direction through the transferring nip portion N while being inserted between the photosensitive drum 1 and the transferring drum 5. A measured result is sent to a controller 11 and the current value I2 for the distance Le is obtained. The controller 11 judges that the current value I2 is lower than the critical current value It which causes the improper transferring and enhances the output voltage so as to obtain the current value I1 sufficient for transferring.
FIG. 9A and FIG. 9B show how the output voltage V and the output current I are enhanced dependently on a distance L from the top of the transferring material-2 by the controller 11 which controls the voltage to be applied to the above described transferring roller 5.
In FIGS. 9A and 9B, a predetermined definite value or a voltage value determined by the above described ATVC system is used as the voltage Va which is to be applied at a timing when the top of the above described transferring material-2 in the moving direction is inserted into the transferring nip portion N formed by the photosensitive drum 1 and the transferring roller 5. On the basis of a fact that the current value I2 is lower than the critical transferring current value It in a condition where the transferring material-2 is inserted in the transferring nip portion N for the distance Le from the top, the controller 40 controls the output voltage from the power source 9 so as to obtain the current value I1 capable of preventing the improper transferring, thereby enhancing the voltage to be applied to the transferring roller 5 to Vb after the distance Le from the top of the transferring material. Accordingly, the ATVC system prevents the improper transferring.
However, the above described conventional example is configured on the premise that a toner image is not formed on the top of the above described transferring material P, though the ATVC system corrects the bias voltage so that the transferring current has an appropriate value while the top of the transferring material P is inserted and conveyed through the transferring nip portion N formed between the photosensitive drum 1 and the transferring drum 5. Furthermore, the conventional ATVC system is configured on a premise that the transferring material P has definite impedance. When a toner image is transferred to the transferring material P which is in a condition where only the top of the transferring material P has high impedance, the conventional ATVC system maintains a transferring current higher than the critical transferring current value It with the top of the transferring material and therefore applies a bias voltage at a corrected level, whereby the ATVC system may supply an excessive current to a portion of the photosensitive drum 1 corresponding to a location of the transferring material P which is other than the top having the high impedance and at which a toner image is not formed.
As a result, the excessive current is supplied locally to the photosensitive drum 1 and the photosensitive drum 1 cannot be charged so as to maintain a dark potential till a next charging time, whereby an image formed next may be partially densified or faded (drum memory).
Using FIG. 10, description will be made of a case where the above described phenomenon may occur due to an impedance difference produced by a copying condition of the top of the transferring material.
FIG. 10 is a diagram showing a relationship between a bias voltage applied to the transferring roller 5 and an output current in a condition where the transferring material P is inserted between the photosensitive drum 1 and the transferring roller 5 in the transferring nip portion N and ready for transferring a toner image, and a current I3 is supplied at a transferring voltage V3 in a condition where a transferring material P3 on which a toner image is not to be formed (a blank copy) is inserted in the transferring nip portion.
In case of a black copy for transferring a toner image over an entire surface, on the other hand, a V-I characteristic is different and impedance is enhanced even for the same transferring material P3. As a result, only a current I3xe2x80x2 is supplied when the same bias voltage V3 is applied. As a result, the ATVC system which uses only a current detecting system recognizes that the transferring material P3 is a transferring material which apparently has impedance higher than that of a transferring material P4 (for blank copy) which has impedance higher than that of the transferring material P3. I4 is a current value at transferring voltage V3 in the condition that a transferring material P4 without forming a toner image is nipped at the transferring nip.
When a toner image is transferred to a top of a transferring material used for monitoring a transferring current, for example, the ATVC system recognizes that the transferring material P3 as a transferring material having high impedance and sets a bias voltage (V3xe2x80x2) at a rather high level. As a result, an excessive current (13xe2x80x3)is supplied to a location of the photosensitive drum 1 corresponding to a location of the transferring material which is other than the top and at which copying ratio is low.
The above described conventional example determines transfer control dependently on impedance of the top of the transferring material as described above and has a possibility to select different control voltages dependently on blank copy and black copy on the top of the transferring material.
When a transferring material only a top of which has high impedance or a narrow transferring material which has high impedance is used, in contrast, the ATVC system judges that a current is at a sufficient level upon detecting a current on the top and determines a voltage to be applied accordingly, thereby hardly preventing the improper transferring to a subsequent location of the transferring material or the narrow transferring material which has the high impedance.
As the image forming apparatus has a higher process speed, a transferring material moves for a longer distance while a current value is detected in a condition where a top of the transferring material is inserted in the transferring nip portion N and a copy ratio cannot be ignored for the control system which monitors a current value in the condition where the top of the transferring material is inserted in the transferring nip portion N.
Though a transferring start timing and a current monitoring timing are determined dependently on a signal from the sensor 8 for synchronizing a top of the toner image on the drum with the top of the transferring material, it is necessary to detect more accurately a moment at which a top of the transferring material is inserted into a transferring nip portion in order to determine a transferring voltage by more accurately by monitoring a transferring current in a narrower area of the top of the transferring material inserted in the transferring nip portion N.
Furthermore, a bias voltage V which is actually applied to the transferring material P inserted in the transferring nip portion N and an output voltage Va which is obtained by controlling so as to supply the predetermined current Ia in the condition where photosensitive drum 1 is in direct contact with the transferring roller 5 are usually in relationship of [V greater than Va].
This is because the drum memory is caused by supplying too high a current in the condition where the photosensitive drum 1 is in direct contact with the transferring roller 5. Therefore, the predetermined voltage V is usually applied at a timing a little later than the moment at which the transferring material is inserted into the transferring nip portion N.
When application of the predetermined bias voltage V to the transferring roller 5 is to be started dependently on a time after detection of the top of the transferring material with the sensor 8 before transferring, however, a high cost and complicated means are necessary for accurately detecting the top of the transferring material. Accordingly, an error of a certain degree is involved by detection of the top of the transferring material and a variation of a certain degree is involved in a time after detection of the top of the transferring material with the sensor 8 till attainment of the transferring material to the transferring nip portion dependently on a kind and a curled condition of the transferring material.
Accordingly, the predetermined voltage V is applied after the transferring material is certainly inserted into the transferring nip portion so that the voltage V which may cause the drum memory will not be applied before the top of the transferring material attains to the transferring nip portion.
In this case, a value of a current which is supplied to the power source 9 in the condition where the top of the transferring material is inserted in the transferring nip portion is monitored for a predetermined time after applying the predetermined voltage V. Accordingly, a time after the transferring material is inserted into the transferring nip portion till the current monitoring is largely variable and a range of the top of the transferring material which is used for the current monitoring is broadened as a process speed is enhanced.
An object of the present invention is to provide an image forming apparatus which is capable of favorably transferring a toner image regardless of impedance of a transferring material.
Another object of the present invention is to provide an image forming apparatus which is capable of using a definite top portion of a transferring material for current monitoring.
Still another object of the present invention is to provide an image forming apparatus which comprises:
an image bearing body which bears a toner image;
a transferring member which forms a nip in cooperation with the above described image bearing body and transfers the toner image on the above described image bearing body to a transferring material;
detecting means which applies a predetermined voltage to the above described transferring member and detects a supplied current when a top of the transferring material is inserted in the nip;
setting means which sets a transferring voltage for a location successive to the top on the basis of an output from the above described detecting means; and
selecting means which selects a transferring voltage to be applied to the transferring material from among the voltage set by the above described setting means and other voltages.
Other objects of the present invention will be apparent from the following description.